1. Field of the Invention
This invention relates to linear, melt processable, crosslinkable, thermotropic wholly aromatic polyesters, to methods of crosslinking such polyesters, to the thus-prepared crosslinked polymers, and to high performance products, such as moldable and extrudable resins, shaped structures, including molded objects, fibers and films, and the like, which incorporate the invention.
2. Description of the Prior Art
a. Wholly Aromatic Polyesters
Linear "wholly aromatic" polyesters, so called because each of their constituent monomers--aromatic diacids, aromatic diols and, on occasion, hydroxyaromatic acids--contributes at least one aromatic ring to the polymer backbone, sometimes referred to as "liquid crystal" polyesters, and various methods for their preparation, have become well known in the art. See, for example, U.S. Pat. No. 4,393,191, issued July 12, 1983 to East and assigned to Celanese Corporation, which contains a copious disclosure of such linear polyesters and methods for their preparation.
As mentioned in the East patent, a considerable advance was made in such polyesters when it was discovered that by a proper choice of monomers linear, wholly aromatic polyesters could be prepared which readily exhibit melt anisotrophy. The formation of an anisotropic melt phase, in turn, permits such polyesters to undergo melt processing to form shaped structures such as molded articles and fibers.
U.S. Pat. No. 4,417,043 discloses polymers, including polyesters, capable of forming an anisotropic melt which include, as part of the polymer chain, self-crosslinking reactants. In the case of the polyesters disclosed, the patent mentions tri- or higher-functional monomeric and polymeric self-crosslinking reactants containing at least three reactive hydroxyl, carboxyl, amino, isocyanate or carbodiimide groups, or combinations thereof. Also mentioned are maleic, fumaric and itaconic acids and their mono- and diesters, but only for use as self-crosslinking reactants in polymers prepared by addition polymerization of monoethylenically unsaturated materials. In all such crosslinkable polymers, crosslinking takes place in the melt. Thus, no more than relatively small amounts of self-crosslinking reactants must be used when preparing crosslinking polymers by the '043 patent's methods to avoid crosslinking the melt to an intractable, unprocessable solid.
b. Polyesters Containing Stilbene and Tolan Derivatives
Various stilbene and tolan (diphenylethyne) derivative-containing polyesters, none of them, however, being linear, melt processable thermotropic, wholly aromatic polyesters, are disclosed in the prior art.
U.S. Pat. No. 2,657,195, for example, discloses linear polyesters obtained by reacting a stilbene dicarboxylic acid compound, such as p,p'-stilbene dicarboxylic acid, and at least one difunctional glycol, diamine or amino alcohol containing an alkylene, cycloalkylene, arylene or aralkylene radical of from about 2 to 16 carbon atoms. Part of the stilbene dicarboxylic acid can be replaced with a dibasic acid compound other than a stilbene dicarboxylic acid, and the polyesters produced can, according to the patentee, be altered in their character " . . . either before or after polymer formation, by reaction with compounds capable of adding to a double bond." The addition of bromine or other halogens, hydrogen halides, hypohalites and organic acids across the double bond, hydroxylation with hydrogen peroxide or potassium permanganate, reaction with maleic anhydride and "crosslinking by known means, such as vulcanization" are all disclosed as examples of such possible alterations. U.S. Pat. No. 2,697,194 contains a similar disclosure, but is restricted to the use of polyalkylene glycols whose alkylene radicals contain from 2 to 6 carbon atoms, such as diethylene and triethylene glycol, as the diol constituents of its polyesters.
U.S. Pat. No. 4,073,777 discloses radiation crosslinkable, water dispersible polyesters containing, as part of their diacid content, an .alpha.,.beta.-unsaturated acid of the structure: EQU R--CH=CH--R'
in which R can be arylcarboxy and R' can be carboxy or arylcarboxy, such as 4,4'-dicarboxystilbene. From 0 to about 80 mol percent of a hydroxycarboxylic acid, 4-carboxycinnamic acid, can also be present.
U.S. Pat. No. 2,856,384 discloses aromatic diacid/aliphatic diol polyesters which include moities derived from 4,4'-dicarboxytolan, with or without small amounts of the 4,3'- and 3,3'-isomers.
U.S. Pat. No. 4,245,084 discloses thermotropic, wholly aromatic polythiolesters which exhibit an anisotropic melt and which can have, as part of their polymer chains, the moiety: ##STR1##
U.S. Pat. No. 4,420,607 relates to high molecular weight copolyesters comprising repeating units from trans-4,4'-stilbenedicarboxylic acid, 0-40 mol % terephthalic acid or 2,6-naphthalenedicarboxylic acid, and 1,2-propanediol. Earlier patents disclosing stilbenedicarboxylic acid-containing polyesters are mentioned as well.
Linear, melt processable, thermotropic, wholly aromatic polyesters differ greatly in their morphology or polymer structure from aliphatic-aromatic or all-aliphatic polyesters. The all-aromatic polyesters in question are, when highly oriented, made up of closely packed polymer chains, as indicated by the high resistance to solvent swelling exhibited by structures, e.g., fibers, formed from such polyesters, and as determined by X-ray crystallography and orientation angle measurements carried out on such structures. Accordingly, such wholly aromatic polyesters would at best be expected to be difficulty crosslinkable if crosslinkable sites were built into the polymer chains for later reaction with external crosslinking agents, as has been done with aliphatic-aromatic and all-aliphatic polyesters, since such external crosslinking agents would be expected to have marginal penetration into such closely-packed structures. Indeed, this fact is underscored by the disclosure of U.S. Pat. No. 4,417,043, whose method of rendering all-aromatic polyesters crosslinkable has, as mentioned, certain inherent deficiencies.
It is, therefore, an object of the present invention to provide novel linear, melt processable, crosslinkable, thermotropic, wholly aromatic polyesters.
It is also an object of the present invention to provide methods of crosslinking such novel linear, melt processable, crosslinkable, thermotropic, wholly aromatic polyesters to produce novel high performance products.
A further object of the present invention is to provide novel linear, melt processable crosslinkable, wholly aromatic polyesters which can first be formed into shaped structures, with no processing problems arising from crosslinking during processing of the melt from which such structures are formed, and then crosslinked to any desired extent.
These and other objects, as well as the nature, scope and utilization of the invention, will become readily apparent to those skilled in the art from the following description and the appended claims.